Protective coating for resistors



Sept. 5, 1944. J. w. JIRA PROTECTIVE COATING FOR RESISTORS Filed June 6, 1941 BY 9 I W'HEZYISR Patented v Sept. 5, 1944 PROTECTIVE COATING FOR RESISTORS Joseph W. Jira, Newbur or to Continental Car Ohio gh Heights, Ohio, assignbon, Inc., a corporation of Application June 6, 1941, Serial No. 396,879

6 Claims.

My invention relates in general to resistance units and more particularly to a protective coating for the resistance units.

An object of my invention is the provision of a protective coating for the resistance units as well as the process for applying the protective coating to same.

My protective coating finds particular utility in connection with the metal film resistors as disclosed in my pending patent application Serial No. 317,154, filed February 3, 1940, now Patent No. 2,281,843, dated May 5, 1942.

Briefly, my metal film resistor comprises a thin film of noble metal of minute thickness externally deposited upon a non-conductive carrier. The thin resistor film of noble metal is extremely thin and difilculty has been experienced in applying a protective coating thereon without deteriorating or damaging the thin film. The film is so extremely thin that the slightest amount of deterioration makes a material difference in the resistance thereof.

Therefore, an object of my invention is the provision of a protective coating for the thin resistor film of noble metal which does not damage the film as the coating is applied thereon.

Another object of my invention is the provision of a vitrified enamel film coating for the thin resistor film and which may b used upon the thin resistor film at a temperature less than the temperature at which a deteriorating action on the thin metal film begins to occur.

Another object of my invention is the provision of a vitrified enamel coating for the thin resistor film of noble metal which coating has substantially the same coefiicient of expansion as that of the non-conductive carrier.

Another object of my invention is the provision of a vitrified enamel coating for the thin resistor film of noble metal which coating is made from materials which are free from lead and iron. I

Other objects and a fuller understanding of my invention may be had from the following description and claims, taken in conjunction with the accompanying drawing, in which:

Figure 1 shows a longitudinal view of a resistance unit embodying the features of my invention, partly being shown in section along the line ll of Figure 2;

Figure 2 is an end view of my resistor unit as shown in Figure 1; and

Figures 3 to 6, inclusive, show the steps by which my protective coating is applied to the thin resistor film of noble metal.

With reference to Figures 1 and 2, my resistor unit comprises a non-conductive carrier it, a thin metal film ll of noble metal of minute or microscopic thickness externally deposited upon the outer surface of the non-conductive carrier, a body of thin metal deposit upon each end portion of the thin metal film H, and conductor terminal members l3 each having a lead ll connected to the body of thin metal deposit l2 upon each end of the resistor. The conductor terminal members I! are wrapped around the body of thin metal deposit 12 and suitably connected thereto such, for example, by mean of solder.

The Figure 3 of the present application is the same as the Figure '7 of my pending patent application Serial No. 317,154, hereinbefore mentioned, and the non-conductive carrier in may be constructed of any suitable material and may comprise a rod or a hollow tube as shown and may be of ceramic material which will withstand thermal shock and which possesses a very low moisture absorbing characteristic. The thin metal film H i made of noble metal and is atomically deposited upon the outer surface of the non-conductive carrier in. The thin resistor film of noble metal II is of minute thickness and thus a considerable amount of difficulty is present in applying a protective coating thereon. The resistor film is so thin that the terminal members It cannot be directly soldered thereto and thus I provide for depositing a body of thin metal I! to each end thereof about which the terminal conductor members l3 may be wr pped and thereafter soldered thereto.

My protective coating is of vitrified enamel and is indicated by the reference character I l in Figure 5. The vitrified enamel coating is originally applied to th outside surface of the thin resistor film II in the form of a slip and may be applied thereto by spraying, dipping or brushing it thereon, or by any other suitable means. The applied vitreous enamel coating prior to being heated is indicated by the reference character I! in Figure 4. The vitreous enamel slip as it is applied to the outer surface of the thin resistor film H is arranged to overlap the body of thin metal deposit i2 and may extend thereover to a place indicated by the reference character It. This overlapping coat prevents any deterioration at the juncture between the body of thin metal deposit l2 and the thin resistor film H. The distance between the reference character" and the end of the body of thin metallic deposit I2 is the place Per cent by weight Feldspar 24 Silica 17 /2 Fluorspar 5 Borax 29 Anhydrous sodium carbonate 8 Sodium nitrate 4 Cobalt 4% Manganese dioxide 1 Cryolite 3 Barium sulphate 3 Black copper oxide V2 In the practice of my invention I make the vitreous enamel free of lead and iron and the frit material may be mixed with clay and water and then milled so that the clay operates as a material to hold the frit material in suspension. The amount of clay added to the frit is of a very small amount and may be in the neighborhood of 2%. Th amount of clay is kept low for the reason that the ultimate fusion temperature of the vitreous enamel slip be maintained at a relatively low value. The flowing temperature at which the applied vitreous enamel is fused to the thin resistor film ii for the example as set forth hereinabove is in I the neighborhood of 1000" F. to 1200 F.

Throughout this range of temperatures, I find that there is substantially no alkaline effect which if allowed to occur would deteriorate the thin resistor film of noble metal. If the flowing temperature is heated much above 1200 F., I find that the alkaline effect tends to deteriorate the thin resistor film which results in a change in ohmic value of the resistor. The thin resistor film of noble metal is microscopically thin and therefore any deterioration whatever of the thin film results substantially in producing an open circuit and causes the ohmic value of the resistor to approach infinity. The alkaline effect appears to result from the fact that the frit is made from materials of which some are alkaline in property. In the example given hereinbefore, the following materials appear to have alkaline properties, namely, feldspar, silica, borax and anhydrous sodium carbonate. It appears that when the vitreous enamel slip which is applied to the thin resistor fihn of noble metal is heated to temperatures much higher than 11200 F. the alkaline materials tend to react chemically with each other and upon the thin resistor film of noble metal to deteriorate the thin resistor film.

In the practice of my invention I provide for heating the non-conductive carrier, the thin metal film of noble metal and the dried vitreous enamel thereon to a flowing temperature of the vitreous enamel'to fuse the vitreous enamel on the thin metal resistor film but below the temperature at which the alkaline eifect would take place to deteriorate the film. The thin resistor film of noble metal H is so thin that its expansion and contraction assumes the characteristic of the expansion and contraction of the non-conductive carrier. Therefore, the vitreous enamel which I fuse to the thin resistor film of noble metal is of such characteristic that it has substantially the same coefllclent of expansion as th non-conductive carrier. This prevents the vitreous enamel coating from spawling or cracking. Accordingly, the resistor unit upon cooling after fusion is not damaged. Also, during operating conditions where the resistor is subjected to electrical loads producing relatively high operating temperatures, the vitreous enamel coating expands and contracts substantially in accordanc with the expansion and contraction of the non-conductive carrier.

Another desirable feature of my vitreous enamel coating is the fact that its fusing temperature is far in excess of the operating temperature of the resistor under load so that the resistance is always maintained at a fixed value. The vitreous enamel coating is so impervious to outside atmospheric conditions that the resistance value is always stable and stays permanently fixed. However, in the manufacture of my resistor units, the resistance value thereof may be altered or changed by helically grooving the resistor units such as shown in'Figures 1 and 6. The helical groove is indicated by the reference character l8 and may be produced by grinding the resistor units and cutting through both the vitreous enamel coating and the thin resistor film of nobl metal. Th groove functions to elongate the resistance path between terminals, thus effecting a change in resistivity.

Although I have described my invention with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of th invention as hereinafter claimed.

I claim as my invention:

1. A protective coating for a thin resistor film of noble metal externally deposited upon a nonconductive carrier comprising a thin layer of vitrified enamel having a fusing point. in the neighborhood of 1000" F. to 1200 F. and having a coefficient of expansion in the neighborhood of that of the non-conductive carrier, said thin layer of vitrified enamel comprising materials which ar essentially feldspar, silica, borax and anhydrous sodium carbonate and in which the proportion by weight of feldspar is in the neighborhood of 24 percent, silica 17 /2 percent, borax 29 percent and anhydrous sodium carbonate 8 percent.

2. A protective coating for a thin resistor film of metal externally deposited upon a non-conductive carrier comprising a thin layer of vitrified enamel having a fusing point at a temperature less than the temperatur at which a deteriorating action on the thin metal film begins to occur and having a coefficient of expan-- sion in the neighborhood of that of the non-conductive carrier, said thin layer of vitrified enamel comprising materials which when combined and fused display a relatively low alkalinity and which are essentially feldspar, silica, borax and anhydrous sodium carbonate and in which the proportion by weight of said materials keeps the fusing point below the temperature at which an alkaline effect would take place to thereby prevent deterioration of the film of metal, said fusing point being in the neighborhood of 1000" F.

and 1200" F.

3. The process of protecting a thin resistor film of metal of microscopic thickness externally deposited upon a non-conductive carrier 'comprising the steps of making a vitreous enamel slip containing a frit made from materials which are essentially feldspar, silica, borax and anhydrous sodium carbonate, proportioning said materials by weight to keep the fusing point below the temperature at which an alkaline effect would take place, applying a thin layer of th vitreous enamel slip to the outside surface of the thin metal resistor film, drying the thin layerof applied vitreous enamel, and heating the non-conductive carrier, the thin metal film and the dried vitreous enamel thereon to a temperature in the neighborhood of 1000 F. to 1200 F. to fuse the vitreous enamel on the thin metal resistor film.

4. The process of protecting a thin resistor film of noble metal of microscopic thickness externally deposited upon a non-conductive carrier comprisin the steps of making a vitreous enamel slip containing a frit made from materials which are essentially feldspar, silica, borax and anhydrous sodium carbonate, proportioning said materials by weight whereby the feldspar is in the neighborhood of 24 percent, silica 17 /2 percent, borax 29 percent, and anhydrous sodium carbonate 8 percent to keep the fusing point below the temperature at which an alkaline effect would take place, applying a thin layer of the vitreous enamel slip to the outside surface of the thin metal resistor film, drying the thin layer of applied vitreous enamel, and heating the non-conductive carrier, the thin metal film and the dried vitreous enamel thereon to a temperature in the neighborhood of 1000 F. to 1200 F. to fuse the vitreous enamel on the thin metal resistor film.

5. A protective coating for a metal resistor element, comprising a thin layer of vitrified enamel having a fusing point in the neighborhood of 1000 F. to 1200 F., said layer of vitrified enamel comprising materials which are essentially feldspar, silica, borax and anhydrous sodium carbonat in which the proportion by weight of feldspar is in the neighborhood of 24 per cent, silica 17 /2 per cent, borax 29 per cent and an hydrous sodium carbonate 8 per cent.

6. A resistor comprising, in combination, a non-conductive carrier, a resistance element composed of a thin film of metal on the non-conductive carrier, and a coating for the resistance element comprising a thin layer of vitrified enamel having a fusing point at a temperature less than at a temperature at which the deteriorating action on the metal resistance element begins to occur, said thin layer of vitrified enamel comprising materials which when combined and fused display a relatively low alkalinity and which are essentially feldspar, silica, borax and anhydrous sodium carbonate and in which the proportion by weight of said materials keeps the fusing point below the temperature at which an alkaline elfect would take place to thereby prevent deterioration of the film of metal, said fusing point being in the neighborhood of 1000 F. to 1200 F.

JOSEPH W. JIRA. 

